Amplifier



Aug. 20, 1940. G, pp I 2,212,205

AMPLIFIER Filed Jan. 4, 1939 INVEN TOR.

GERARD EPP A TTORNEY.

Patented Aug. 20, 1940.

. -by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware i Application January 4, 1939, Serial No. 249,222

In Germany January 4, 1938 I 7 Claims.

This invention relates to amplifying circuit arrangements for amplifying a given frequency band, particularly widefrequency bands including .the low frequencies, in such a manner that the amplification is independent. of frequency so that uniform amplification throughout the band is obtained.

Wide band amplifiers having uniform amplification or frequency-independent characteristics are of considerable importance in electrical arts and especially injtelevision. Hitherto the construction of such amplifiers have been exceedingly difficult andcostly and in many cases could only provide uniform amplification at the cost of non-uniformtime'delay of signals of different frequencyknown. as non-linear phase'respons'e characteristic. This invention provides a relatively simple and cheap means of obtaining uniform amplification through a wide band of frequencies and at the same time preserving uniform phase response.

Accordingly, it is one object of this invention .to provide an improved wide band amplifier.

Another object of the invention is to provide a simple wide band amplifier having uniform amplification and time delay.

Other objects of my invention will become apparent upon reading the detailed description together with the drawing. 1

In the drawing, Fig. 1 shows in schematic diagram form, the connections of an amplifier and circuit elements embodying the invention,

While Figs. 2-5 show in circuit diagram form modifications of the embodiment of the invention shownin Fig. 1.

It is known that it is possible to obtain amplification which is independent of frequency within wide limits by means of an amplifying valve whose anode circuit includes a resistance across which the amplified voltage occurs. In such a 137 other impedances'that may influence the frequency relation to a greater or less extent. If a screen grid valve is used as the amplifier valve the screen grid conductor generally includes an impedance dependent on frequency which is constituted, for example, by a resistance and a parallel condenser and serves for thesecuringof the desired screen grid voltage and for smoothing the screening grid voltage from the voltage supply feeding the anode. The alternating voltages to be amplified produce a voltage drop across this impedance which is" dependent on frequency and which, due to the control action of the screening grid on the anode current, brings about a voltage circuit arrangement there are, however, frequent drop across the resistance in the anode circuit which, is dependent on frequency. The amplification is consequently dependent on frequency.

The lead connecting the anode and the screen grid to the source from which the anode and screen grid voltages are obtained often includes in addition an impedance, for example the internal; impedance, of said source, or a resistance withparallel-condenser, which serves for smoothingby-passing -or uncoupling. This impedance also influences the variation of the amplifier with frequency. v

According to the invention, in order to obtain an amplification which is substantially invariant with frequency in spite of the presence of one or both of the two above-mentioned impedances, the "anode circuit of the amplifier valve includes, in series with the anode resistance, impedances which are respectively proportional to the impedance in the screen grid conductor and/or to the impedance in the common conductor by which the anode "and the screen gridare connected to the "source: of voltage. I

In addition it is often desirable that the amplifier should be insensitive to potential variations of the source ofvoltage." According to a further feature of the invention, as willbe shown hereinafter, this may be ensured by making the resistance in the anode circuit of the amplifier valve equal to the reciprocal of the slope of the anode current-screen grid voltage characteristic. In this case theimpeda'nce in the anode circuit which is proportional tothe impedance in the common lead connecting the anode and the screen grid to the source; of voltage may be omitted.

Fig. 1 of the drawing shows a schematic diagram of connections of an amplifier embodying the invention. This circuit arrangement comprises an amplifier valve of the pentode type whose screen gridfconductor 'includes an impedance ZS. The common lead by which the anode and the screen grid are connected to the source of voltage Ea includes an impedance Zc;

according'tq the invention the resistance Ra in the anode circuit has connected in series therewith two impedances aZS and fiZc which are proportional respectively to Z5 and Z0. As may be established by calculation or measurement, the ratio between the amplified voltage e2 and the unamplified voltage 21, or in other words the amplification, is independent of frequency if the proportionality-factors cc and ,8 are chosen in accordance with the following'conditions:

q designating the ratio between the screen grid alternating current is and the anode alternating current ia, which ratio may be deemed constant, and SS designating the slope of the anode current-screen grid voltage characteristic.v

If on and p are chosen in accordance with the above equations the amplification is given by the expression,

and F For networks of Z5 and Z other than those just mentioned the impedances in the anode circuit which are proportional to them may be readily determined in a similar manner.

In order that the circuit arrangement shown in Fig.' 1 or 2 may become insensitive to potential variations of the source of voltage Ea, the resist ance Ra is preferably made equal to the reciprocal of the slope of the anode 'current-screen grid voltage characteristic, consequently In this case, due to the control action of the screen grid on theanode current, a potential variation of the source of voltage Ea. brings about an identical but oppositely directed potential variation at the resistance Ra so that the latter compensates exactly for the former. No potentialvariations, therefore, occur between the output terminals A1 and This is not altered either when further impedances are included between the terminals A1 andjAz, which may be de- I sira-ble for reducing the total impedance included in the anode circuit, since the proportioning influence of the anode-cathode capacity of the amplifier valve on the amplification of the higher frequencies is greater than can be conveniently handled. In this case, suitable impedances may be arranged between the terminals A1 and A2 so that the total impedance included in the anode circuit is reduced and the influence of the anode cathode capacity of the amplifier valve consequently decreases.

Such a circuit arrangement is shown inFig. 3 in which it is premised that the form of the impedance Za. corresponds to the data above given. An optional impedance Z1 is connected, in parallel with Za, in series with impedances The output voltage 32 is obtained from the two The amplification of this circuit arrangement is found to'be given by V and if Za is consequently chosen in accordance with the above equation this amplification is again independent of frequency.

By suitable choice of the factor 11 the total impedance included in the anode circuit may be reduced to a value as small as desired so that the influence of the anode-cathode capacity is reduced.

It may be observed that if the proportionality factor 3 becomes equal to zero and the impedance {3Z0 is consequently omitted.

The impedances Z and Z0 need not always be present in the circuit arrangement at the same time. Thus, for example, it is often possible to omit the impedance Zc or, if it forms the internal impedance of the source of voltage Ea, to neglect it. In this case, however, it may be desirable to include in the anode circuit an impedance Zb which isconstituted, for example, as usual by a IGSiSt8.C-Ra with parallel .condenser Cb, as is shown in Fig. 4. Again, as is clearly evident, the amplification is independent of frequency when a proportional impedance is provided in the screen grid circuit.

The-impedances Zb and lXZs of Fig. 4 are preferably so transformed as to also assist in smoothing the anode voltage. This leads to the circuit arrangement shown in Fig. 5 in which resistances R1 and R2 are united with the condensers C1 and C2 to form a filter. lation is concerned, this circuit. arrangement is equivalent to that shown in Fig. 4 if R1, R2, C1 and C2 are such that they satisfy the following conditions:

In the foregoing we assumed the impedances Z5, Z0, OtZs and 5Z0 to be associated with the same amplifier stage. It is however, obvious that when the impedances ZS and Zc are associated with an the impedances DtZs and BZc are associated it is also possible to obtain amplification that is independent of frequency. In this case, the proportionality factors must of course be chosen in accordance with the particularities of the tubes and circuit arrangementsin use.

It is also possible simultaneously to compensate, inthe anode circuit of amplifier the influence on the frequency relation of the impedances Z5 and Z0 at the same stage and also of the corresponding impedances at one or more of the preceding and/or following stages.

Having described my invention, What I claim is: i 1..-A wide band amplifier having uniform So far as the frequency reamplifier stage different from that with which amplification comprising an electron discharge tube having a cathode, a control grid, 2. screen grid and an anode, a supply source, a connection between the cathode and supply source, a first impedance connected between said screen grid and a second impedance, said second impedance being connected to the cathode through the supply source, an anode circuit comprising a resistor, a third impedance proportional to said first impedance, and a fourth impedance proportional to said second impedance connected in series, and means to impress signal voltagebetween the control grid and cathode.

2. An amplifier as claimed in claim 1 and wherein the resistance in the anode circuit is substantially equal to the reciprocal of the slope of the anode current-screen grid voltage characteristic of the tube.

3. An amplifier as claimed in claim 1 and where all of said impedances comprise a resistor connected in parallel with a condenser.

4. An amplifier as claimed in claim 1 and comprising in addition, connected in parallel with said anode circuit, a circuit including a fifth impedance connected in series with a sixth and a seventh impedance, said sixth impedance being proportional to said fifth impedance and said seventh impedance being proportional to the total impedance of said anode circuit.

5. An amplifier as claimed in claim 1 and wherein said second impedance is zero and comprising in addition, a first auxiliary impedance connected in said anode circuit, and a second auxiliary impedance connected in series with said first impedance, said second auxiliary impedance being proportional to said first auxiliary impedance.

6. A wide band amplifier having uniform amplification comprising an electron discharge tube having a cathode, a control grid, a screen grid and an anode, a supply source, a connection between the cathode and supply source, a first impedance connected between said screen grid and a second impedance, said second impedance being connected to the cathode through the supply source, an anode circuit comprising a resistor, a third impedance proportional to said first impedance, a fourth impedance proportional to said second impedance connected in series, means to impress signal voltage between the control grid and cathode, and output terminal means connected to said anode and said cathode.

'7. A wide band amplifier having uniform amplification comprising an electron discharge tube having a cathode, a control grid, a screen grid and an anode, a supply source, a connection between the cathode and supply source, a first impedance connected between said screen grid and a second impedance, said second impedance being connected to the cathode through thei supply source, an anode circuit comprising a resistor, a third impedance proportional to said first impedance, a fourth impedance proportional to said second impedance connected in series, a circuit connected in parallel with said anode circuit comprising a fifth impedance connected in series with a sixth and seventh impedance, said sixth impedance being proportional to said fifth impedance and said seventh impedance being proportional to the total impedance of said anode circuit, means to impress signal voltage between said control grid and said cathode, and output terminal means connected to the junction of said fifth and said sixth impedance and to said cathode.

GERARD HEPP. 

